1. Field of the Invention
The present invention relates generally to fiber optic interconnections. This invention relates in particular to coupling light between multicore fibers and elements with optical waveguides.
2. Description of the Related Art
Multicore fibers have a multitude of cores embedded in a common cladding. As such, they can transport more bandwidth per fiber than standard fibers with a single core and hence lead to a potential cost reduction. Multicore fibers have been cited in the literature as early as 1979 and various newer types of multimode and single-mode multicore fibers have been proposed over the years, but they have to date not been a major commercial success. This is largely due to the absence of a simple and low-cost optical connector to couple between multicore fibers or between optoelectronic elements and multicore fibers. Therefore, a need exists for a low cost method to couple light to and from multicore fibers.
Drawbacks of existing schemes to couple between optoelectronic devices and multicore fibers are discussed below. Some coupling schemes use either butt coupling or a two-lens optical relay. Custom optoelectronic device arrays (lasers, photodiodes) are required that match exactly the geometry of the multicore fiber. Rotational alignment is critical and has not been successfully addressed in a passive alignment process. Active alignment of each individual fiber would be costly. Using two-lens optical relays, efficient coupling could be possible, but density between neighboring fibers (typically 125-250-μm diameter) is limited, thereby giving up some of the advantage of multicore fibers. Optical crosstalk is a concern as the fiber cores are typically spaced by only 30-40 μm. Apertures may help to avoid crosstalk, but at the expense of higher insertion loss.
Several connector concepts have been proposed to couple light from a multicore fiber into multiple single core fibers. All of these concepts achieve coupling of light and rotational alignment between a multicore fiber and several single core fibers, either through a mechanical fixture or through splicing. However, none of these concepts demonstrates coupling between multicore fibers and optical waveguides.
Another concept describes active fiber alignment, using a light source at an input of a multicore fiber and a light detector at an output. However, coupling multicore fibers to optical waveguides is not disclosed. Also, this concept involves active fiber alignment, based on active rotation (and optional translation) of the fiber for the highest signal at the detector. Unlike passive alignment, active alignment of multicore fibers is a time-consuming and costly process that is unsuitable for high-volume applications demanding ultra-low cost connections.
Another related art includes a method for connecting a multicore fiber to optical devices. However, this is also an active fiber alignment device and no means for passive rotational fiber alignment is described. Also, coupling to optical waveguide elements is not disclosed, and the fiber type described in this work is a photonic crystal fiber, which is a very specific sub-type of multicore fibers with limited applications.
Other related art describes a multicore fiber coupling system which supports use of coupling to an optical waveguide. This related art describes a modulated ring laser coupled to a fiber using an intermediate waveguide. It envisions coupling to a multicore fiber, but only out of plane.
In-plane coupling between waveguide substrate and fiber ribbons is described by another related art. However, in-plane coupling is limited to a substrate with a single plane. That is, coupling between a substrate with a single layer of waveguides and an optical fiber ribbon. Also, no active or passive means for alignment of the multicore fiber to the substrate with waveguides is described.